The about $1099 Voigtlander Heliar-Hyper Wide 10mm f/5.6 Aspherical for Sony E mount is the widest full frame lens on the market as far as I am aware. It is also available in M-Mount. Indeed, it is a tweaked rangefinder design.
This scene in my review of the Voigtlander 10mm f/5.6 Aspherical shows what might be a valid use for a 10mm lens: trying to make an image with extreme perspective in a tight space.
Image sizes up to 28 megapixels, with crops from f/5.6 through f/13.